Topological defects of various kinds are predicted
to have occurred during phase transitions in the early Universe. The
exact character of the defect that would have been produced depends
exactly on the nature of the phase transition and the configuration of
the fields involved in spontaneous symmetry-breaking. Their existence
can be argued on general grounds because of the existence of horizons
in cosmology. If a phase transition happens more or less
simultaneously in all regions of the Universe, then there is no
possibility that regions separated by more than the scale of the
cosmological horizon at the time can exchange light signals. Whatever
the configuration of the vacuum state of whatever field is undergoing
the transition in one region, the state in a different, causally
disconnected part of the Universe would be expected to be
independent. This incoherence of the field would have resulted in
defects, much like the defects that appear when liquids are rapidly
cooled into a solid phase. Solids formed like this tend to have only
short-range order within domains separated by defects in the form of
walls. Other types of defect are possible, depending on the type of
phase transition. Some of the cosmological defects that have been
suggested are described overleaf.

Monopoles (sometimes called magnetic monopoles) are hypothetical
point-like defects in the fabric of spacetime, produced in the early
Universe according to some grand unified theories (GUTs). No monopoles
have yet been detected in the laboratory. These objects are
historically important (even though their existence is entirely
speculative), because the inflationary Universe model was originally
suggested as a means of reconciling the present lack of observed
monopoles with GUT theories. The rapid expansion of the Universe
associated with inflation simply dilutes the number of monopoles
produced in the phase transition to an unobservably small value.

Cosmic strings are one-dimensional (line-like) defects, slightly
similar to the vortex tubes that can be produced in liquid helium
phase transitions. If produced in the framework of a GUT, such a
string would be about 10-31 metres thick, and have a mass of
about ten
million solar masses per light year. Because of their strong
gravitational effect on nearby matter, it has been suggested that
cosmic strings might play a significant role in cosmological structure
formation by generating large enough primordial density
fluctuations. It is now generally accepted, however, that this of
large-scale structure and the fluctuations in the cosmic microwave
background radiation disagree with the predictions of cosmic-string
theory.

Domain walls would be two-dimensional (sheet-like) defects. In
essence, they are wall-like structures in which energy is trapped,
rather like the Bloch wall formed between the Weiss domains in a
ferromagnet. Any theory of the fundamental interactions that predicts
large numbers of domain walls would predict a highly inhomogeneous
universe, contrary to observations, so these particular defects are to
be avoided at all costs.

Cosmic textures are by far the hardest kind of defect to visualise;
they involve a kind of twisting of the fabric of spacetime. Like
cosmic strings, these entities have been suggested as possible sources
for the primordial density fluctuations, but they have fallen out of
favour because they fail to reproduce the so-called Doppler peak seen
in observations of the cosmic microwave background radiation (see
Sakharov oscillations).